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2352 - Equal Row and Column Pairs (Medium)

https://leetcode.com/problems/equal-row-and-column-pairs

Problem Statement​

Given a 0-indexed n x n integer matrix grid, return the number of pairs (Ri, Cj) such that row Ri and column Cj are equal.

A row and column pair is considered equal if they contain the same elements in the same order (i.e. an equal array).

Example 1:

Input: grid = [[3,2,1],[1,7,6],[2,7,7]]
Output: 1
Explanation: There is 1 equal row and column pair:
- (Row 2, Column 1): [2,7,7]

Example 2:

Input: grid = [[3,1,2,2],[1,4,4,5],[2,4,2,2],[2,4,2,2]]
Output: 3
Explanation: There are 3 equal row and column pairs:
- (Row 0, Column 0): [3,1,2,2]
- (Row 2, Column 2): [2,4,2,2]
- (Row 3, Column 2): [2,4,2,2]

Constraints:

  • n == grid.length == grid[i].length
  • 1 <= n <= 200
  • 1 <= grid[i][j] <= 10^5

Approach 1: Hash Map​

Written by @wingkwong
class Solution {
public:
    int equalPairs(vector<vector<int>>& g) {
        int n = g.size(), ans = 0;
        // count the frequency of a vector
        map<vector<int>, int> m;
        // check all rows
        for (int i = 0; i < n; i++) {
            vector<int> tmp;
            for (int j = 0; j < n; j++) tmp.push_back(g[i][j]);
            // in example 1, we got {3,2,1}, {1,7,6}, {2,7,7}
            m[tmp] += 1;
        }
        // check all cols
        for (int j = 0; j < n; j++) {
            vector<int> tmp;
            for (int i = 0; i < n; i++) tmp.push_back(g[i][j]);
            // in example 1, we got {3,1,2}, {2,7,7}, {1,6,7}
            // since {2,7,7} has frequency of 1, we add it to answer 
            // otherwise, it would be adding 0
            ans += m[tmp];
        }
        return ans;
    }
};